Accurate interactive acoustic plate

ABSTRACT

A device for acquiring the position co-ordinates of a source of mechanical waves optionally generated by impacting the surface of a plate (PLQ) of finite dimensions. A set of acoustic sensors (PZT 00  to PZT 11 ) each formed by a pair of piezoelectric transducers (PZTa, PZTb) face each other on either side of the plate, the device includes a processor for determining the co-ordinates of the source by analyzing the difference in propagation time of the acoustic waves generated by the source to the different sensors. The processor combined with each sensor (PZT 00  to PZT  11 ) a respective electronic circuit includes a digitized mounted in cascade for digitizing the amplified signal around a predetermined frequency, associated with a mechanism for limiting the digitization to a time window starting before the acoustic waves reach a sensor and ending when the acoustic waves have reached said sensor.

The present invention deals generally with devices for interactivecommunication between a user and a machine.

More precisely, the invention relates to a device for gathering and forprocessing acoustic waves transmitted by a user or a sensor to a plateserving as interface with a machine, said device analyzing thepropagation times of the acoustic waves in the plate so as in particularto measure the coordinates x_(r), y_(r) of impacts on the surface of theplate.

Patent WO96/11378 discloses a device for acquiring coordinates x_(r),y_(r) of that point of a rigid plate from which a source emits,pointwise, wave packets, by analyzing the propagation time of the wavesin the plate in two directions x and y of the plate.

Also, French patent 9816229 of 22 Dec. 1998 discloses an acquisitiondevice where two pairs of transducers are respectively associated witheach direction x, y, the position of the source along each directionbeing determined by measuring the differential of the arrival times atwhich the wave packets reach the two respective pairs of transducers ofsaid direction.

These two acquisition devices are especially well adapted to theselective detection of an acoustic Lamb mode in an isotropic glassplate. In particular, the device of patent 9816229 proposes a processfor quantizing the intensity of an impact enabling it to attain anaccuracy of 1% in the position measurement. The present invention showshow to further improve this result, this turning out to be necessary ifone wishes to be able to point reliably at zones of the size of a squarecentimeter on a plate of four to ten square meters. Furthermore, thedevices described in the abovecited patents are rather unreliable whenthe plate is a laminated glass, that is to say is an assemblage of twoor more single-strength or annealed or tempered sheets of glass,intimately stuck together by interposition of one or more polymer filmssuch as polyvinyl butyral. For safety reasons, the use of laminatedglasses is a condition which is ever more frequently demanded in thepublic arena. In case of breakage, the polymer constitutes areinforcement to which the glass splinters remain stuck, thus enablingthe laminated glass to ensure residual protection before replacement.

The use of laminated glass for producing acoustic panes operating on theprinciple of the locating of impacts by measuring the propagation timeof ultrasound plate modes requires an improvement to the devicesdescribed in the above patents.

The aim of the present invention is thus to improve the quality and thereliability of reception of acoustic waves in a device adapted both toisotropic single-strength or laminated plates. In particular, theinvention will show how to improve the accuracy of measurement by makingthe latter yet more independent of the intensity of the shock.

Another aim of the present invention is also to improve the ergonomicsof the device in particular through automated calibration of the platemaking it possible to allow for the effects of temperature on thevelocity of propagation of the acoustic waves and hence on the accuracyas well as through the establishing of a simpler and faster homotheticcorrespondence between the real coordinates of an impact on the plateand the screen coordinates of a graphical interface projected directlyonto the plate with the aid of a video projector, it being possible forthe size of said graphical interface to vary rapidly as a function of adesired or accidental movement of the video projector.

The invention is also concerned with improving the communication meansmade available to the user in order to emulate according to theprinciple of activation by an impact, the manner of operation of apointer of mouse type or of an alphanumeric keyboard required forsearching for information on the Internet.

The invention shows finally how to mask the noise of impacts on theglazing through a process of superimposing a synthesized sound on thesound emanating from an impact.

To achieve these aims, the invention proposes a device for acquiringcoordinates of points of interaction of an acoustic source with thesurface of the plate, optionally laminated, of finite dimensionscomprising a set of acoustic sensors each formed of a pair ofpiezoelectric transducers situated facing one another on either side ofthe plate, the device comprising processing means for determining thecoordinates of said point of interaction by analyzing the difference inpropagation times of the acoustic waves emitted by the source to thevarious sensors, a device characterized in that the processing meanscomprise in association with each sensor a respective electronic circuitcomprising in cascade, means for performing a broadbandpreamplification, means of selective amplification centered on a firstspecified frequency, means for detecting the head of the wave packet andfor sampling the signal over a time window encompassing the head of thewave packet as well as means for switching the sensors to emitter or toreceiver so as to determine the velocity of propagation of theultrasound waves or else to carry out an integrity check on the plate,as well as means for calibrating the acquisition system in an automatedmanner which is therefore simple for the user.

Preferred but nonlimiting aspects of the device according to theinvention are the following:

-   -   the sensors are four in number and the piezoelectric transducers        of each sensor are disks of piezoelectric ceramics stuck to        either side of the plate, in such a way that four sensors form        the vertices of a rectangle whose center constitutes the origin        of the coordinates,    -   the rectangle defined by the sensors is divided into four        quadrants, each quadrant being associated with a trio of sensors        which are closest to the center of the quadrant responsible for        detecting the coordinates of an impact in this quadrant with a        better measurement accuracy than that which would be obtained        with the other trios,    -   the piezoelectric ceramic disks have a silver-plated lapover        enabling the electrical connections to be made on the same face        of the disk,    -   the piezoelectric ceramics are of ferroelectric type,    -   the silver-plated lapover is preferably cylindrically symmetric.        When the silver-plated lapover is not cylindrically symmetric,        it is positioned in such a way that the sensor has the most        omnidirectional possible sensitivity,    -   the electrical connections of the two piezoelectric transducers        of each sensor are linked in parallel, the polarization vectors        being arranged symmetrically with respect to the midplane of the        plate in such a way that the slower antisymmetric modes are        discouraged and the faster symmetric modes are favored,    -   the locating of a point of impact with a laminated glass        consisting of an assemblage of two identical plates, each of        thickness e, bonded together by a polymer film, consists in        extracting the ultrasound frequency component fc satisfying the        rule: f_(c).e=1.2 MHz.mm, said frequency being generated by the        impact of a hard object such as the flat of a fingernail, a        metal key, a hard plastic rod,    -   the locating of a point of impact on a plate in one of the four        quadrants defined by the pair of bits (g_(y), g_(x)) consists in        measuring the difference in flight times between two sensors,        taken from among a trio of sensors, defining a first direction        and two sensors, taken from among the same trio of sensors,        defining a second direction perpendicular to the first, so that        the cartesian coordinates of the point of impact (x_(r), y_(r))        on the plate are given by the formula: $\begin{matrix}        {X_{r} = {{( {- 1} )^{g_{x}}\frac{\quad{\Delta\quad{txg}\{ {q\sqrt{p^{2}{v^{2}( {{4p^{2}} - {v^{2}\Delta\quad{txg}^{2}}} )}( {{4p^{2}} + {4q^{2}} - {v^{2}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}^{2}} )\Delta\quad{{tyg}^{2}( {{4q^{2}} - {v^{2}\Delta\quad{tyg}^{2}}} )}}} \}}}{4p\quad\Delta\quad{{tyg}( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{tyg}^{2}}} )}} )}}} +}} \\        {( {- 1} )^{g_{x}}\frac{\quad{\underset{\quad}{\quad}\Delta\quad{txg}\quad p^{2}v^{2}\Delta\quad{{tyg}^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{{tyg}( {{{- \Delta}\quad{txg}} + {\Delta\quad{tyg}}} )}}} )}}}{4p\quad\Delta\quad{{tyg}( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{tyg}^{2}}} )}} )}}}        \end{matrix}$ $\begin{matrix}        {Y_{r} = {{( {- 1} )^{g_{y}}\frac{q\quad v^{2}\Delta\quad{{txg}( {{{- 4}p^{2}} + {v^{2}\Delta\quad{{txg}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}}} )}\Delta\quad{tyg}}{4( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}Ä\quad{tyg}^{2}}} )}} )}} +}} \\        {( {- 1} )^{g_{y}}\frac{\sqrt{p^{2}{v^{2}( {{4p^{2}} - {v^{2}\Delta\quad{txg}^{2}}} )}( {{4p^{2}} + {4q^{2}} - {v^{2}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}^{2}} )\Delta\quad{{tyg}^{2}( {{4q^{2}} - {v^{2}\Delta\quad{tyg}^{2}}} )}}}{4( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}Ä\quad{tyg}^{2}}} )}} )}}        \end{matrix}$    -   where p and q are positive numbers respectively representing the        abscissa and the ordinate of the position of the sensors with        respect to the center of the rectangle defined by the sensors,        the speed of the plate mode detected, that is to say, for a        laminated glass, that of the fastest mode, Δt_(xg),        (respectively Δt_(yg)) the difference in the propagation times        of the wave packet generated by the impact between the sensors        of the first pair situated in the half-rectangle g_(x)        (respectively of the second pair situated in the half-rectangle        g_(y)),    -   the determination of the trio of sensors associated with a given        impact is performed according to an algorithm which searches for        the largest difference in flight times available between the        sensors in two perpendicular directions,    -   said electronic circuits associated with the sensors comprise in        cascade a broadband preamplifier stage, an amplifier stage        selective in the ultrasound band, a squaring stage, a peak        detector stage, an integrator stage, a stage for adaptation to a        logic level constituting a synchronizing signal “SYNC” intended        on the one hand for the approximate calculation of the position        of an impact, on the other hand to order the stoppage of the        digital recording of the signal associated with the sensor, on        the other hand for the starting of the sound enrichment        procedure described hereinbelow,    -   the electronic circuits associated with the respective sensors        comprise downstream of said means of selective amplification,        synchronous analog digital converters associated with FIFO        (first in-first out) memories of sufficient depth to digitize        the equivalent of several acoustic periods of the selected        frequency, in such a way as to furnish a digital recording        starting before the arrival of the head of the wave packet, said        digitization of the signal being characterized by a sampling        frequency of at least 1 MHz,    -   the difference in flight times between the sensors is deduced,        on the one hand, from the time intervals between the        synchronization signals, on the other hand, from the time        intervals separating the synchronization signals from the heads        of the wave packets,    -   the electronic circuits associated with the respective sensors        comprise upstream of said means of selective amplification a        bypass to means of digitization, analysis and frequency        enrichment of the audible acoustic signal generated by the        impact on the plate, as well as means for reconverting the        enriched digital signal into an analog signal and sending it to        loudspeakers so as to mask the nuisance caused by the impact in        the form of a more pleasant sound reproducing for example the        noise of a percussion instrument within a symphonic composition,        or the noise of an animal or of a natural event, said means of        enrichment being implemented at the very instant at which the        first of the four synchronization signals is activated,    -   the measurement of the time interval separating a        synchronization signal from the head of the associated wave        packet consists in determining the instants of zero-crossing of        the digitized signal by backtracking in time from the instant of        switching defined by SYNC, while a test on the sum of the        amplitudes between the zero-crossings, that is to say a test on        the mean value of the signal per half-period, makes a decision        regarding the continuation of the algorithm. When the mean value        over a half-period is equal to the output value from the sampler        in the absence of any signal, to within a threshold discrepancy,        the algorithm is stopped, and the mean value is regarded as        being the origin instant t_(HD) of the packet,    -   one at least of the sensors is able to be switched into an        emitter of an ultrasound wave packet detected by the other        sensors whose positions are known so as, on the one hand, to        measure in an automated manner the temperature-dependent        velocity of propagation of the ultrasounds, on the other hand,        to trigger an integrity test on the plate, by measuring the        difference in propagation times of the wave packet between the        various sensors,    -   the management of all the steps of measurement, processing and        transfer of the data by a serial communication interface, either        with an LCD display, or with an interface for musical        instruments (MIDI interface), or with a more powerful computer        such as a microcomputer via its RS232 or USB port is supervised        by a microcontroller,    -   the device constitutes a peripheral interface with a machine        which receives the signals emanating from the processing means        and which as a function of said signals orders the execution of        files contained in the computer by various peripherals of known        types linked to the computer, such as for example in a        nonlimiting manner a video projector, light sources,        loudspeakers, a printer, or else automatic tackle controlling a        mechanical action such as the closing of a protective curtain,    -   the system is furnished with software means for emulating the        manner of operation of another peripheral such as a pointer of        mouse type, or a keyboard and constitutes a peripheral interface        with computer which as a function of the signals emanating from        the processing means orders the execution of files contained in        the computer or, via a modem or a network card, the execution of        files contained remotely on a server,    -   the computer is associated with a flat screen of large        dimensions or, better still, with a computerized video projector        which projects onto a portion referred to as the screen zone of        the surface of the plate, at least one of whose faces is made of        frosted glass, the information available from an Internet        portal,    -   the frosting of the surface can be replaced by the laying of a        scattering film, possibly in combination with a Fresnel lens        playing the role of directional spyglass, that is to say of        directional concentrator of light enabling the graphical        interface to be used even in full daylight,    -   the device is furnished with software means making it possible        to depict, reduce or move a keyboard on the screen zone,    -   the device is furnished with software means allowing fast and        simplified automated establishment of a homothetic        correspondence between the real physical coordinates in        millimeters of an impact and the graphical coordinates in pixels        of this impact when it is produced inside the graphical zone.        The axes of the graphical and real coordinate systems are        parallel and said software means involve the following        operations:        -   displaying by the software of a target at various positions            with known graphical coordinates and measuring of the            corresponding real coordinates. For example, a first target            is displayed at N₀(i, j) where i and j are screen            coordinates, ready of the screen origin. This target is            displayed on the acoustic plate at the real coordinates            N₀(x_(a), y_(c)). An impact carried out at the place where            the target appears makes it possible to gather these real            coordinates via the acquisition card. A second target with            screen coordinates N₁(k, l) is then displayed ready of the            maximum coordinates of the screen. The corresponding real            coordinates N₁(x_(b), y_(d)) are obtained through an impact            opposite the target.        -   the graphical coordinates (N_(qx), N_(qy)) of any point Q            with real coordinates (x_(r), y_(r)) may then be deduced            from the formula: $\begin{matrix}            \begin{Bmatrix}            {N_{qx} = {i + {( {k - i} )\frac{( {x_{r} - x_{a}} )}{( {x_{b} - x_{a}} )}}}} \\            {N_{qy} = {j + {( {l - j} )\frac{( {y_{r} - y_{d}} )}{( {y_{c} - y_{d}} )}}}}            \end{Bmatrix}            \end{matrix}$        -   a reference target is displayed a last time at the center of            the graphical screen. The impact carried out opposite the            target is converted into screen coordinates according to the            above formula. The calculated position is compared with the            reference position. If the discrepancy is below a certain            threshold, the calibration operation is validated. Otherwise            it is repeated.    -   the device is furnished with software means such that the        portion of the plate which does not serve as a screen is        nevertheless interactive and is configured as an extension of        the screen zone, in particular an impact produced to the left        (respectively, to the right, above, below) of the screen zone        moves the content of the screen toward the right (respectively        to the left, below, above), thus making it possible to read a        document of much greater size than the size of the screen zone.    -   the device is furnished with software means such that the        portion of the plate which serves as a screen is regarded as a        special zone making it possible to quit or to switch from any        software application managing the execution of groups of        programs associated with various zones of the plate which are        situated off-screen.    -   conversely, the system is furnished with software means such        that any impact produced on the plate outside the screen zone is        associated with the execution of a chosen application managing        the workspace situated off-screen, such as for example the        application where, on the basis of the homothetic correspondence        established between the pixels of a digital photograph of the        acoustic plate and the physical coordinates of these pixels on        the acoustic plate, groups of programs are executed following an        impact on a given zone of the plate.

Other aspects aim and advantages of the present invention will becomemore apparent on reading the following detailed description of preferredembodiments thereof, given by way of nonlimiting example and made withreference to the appended drawings in which:

FIG. 1 is a diagrammatic view of a rectangular plate giving the locationand the coordinates of the sensors, the real and graphical referenceframes defined, as well as the real and graphical coordinates of threeimpacts on the plate.

FIG. 2- a is a screen capture of a dialog window making it possible toconfigure the graphical interface and to define the authorized zone inwhich the impacts are interpreted as events of a pointer of mouse type.

FIGS. 2- b to 2-f are screen captures of the process for calibrating thegraphical interface making it possible to establish the homotheticcorrespondence between the physical coordinates and the screencoordinates of the portion of the plate playing the role of screen.

FIG. 3 a is a screen capture of the main keys of an alphanumerickeyboard occupying almost the entire width of the graphical screen andalmost ⅓ of its height. The keyboard is enriched with two supplementarykeys making it possible to move it or to reduce its size to two floatingkeys according to FIG. 3 b.

FIG. 4 is a timing diagram of the main steps of measuring the differencein transit times between two sensors.

FIG. 5 a is a block diagram of the general architecture of theacquisition card designed on the basis of analog and digital circuitswith, in particular, the use of a microcontroller capable of carryingout the processing of the data for the calculation of the instant ofarrival of the wave packet and the transferring of the data by a serialcommunication protocol to a microcomputer or another device.

FIG. 5 b is a circuit diagram of the PMP module of FIG. 5 a andrepresenting a generator delivering a high voltage HT using a diodepump, synchronous with the microcontroller's system clock.

FIG. 6 is a diagrammatic sectional view of the symmetric arrangement ofthe piezoelectric transducers of a sensor intended for detecting themode S₀ corresponding to the fastest vibrations propagating in alaminated glass.

FIGS. 7 a and 7 b show the signals emanating from the transducers ofFIG. 6 in response to an impact 70 cm away (FIG. 7 a) and 130 cm away(FIG. 7 b), after selective amplification around 100 kHz.

FIG. 7 c shows in greater detail the loss of symmetry observed on themodes S₀ detected by the transducers PZTb and PZTa of FIG. 6 in the caseof a laminated glass consisting of an assemblage of two plates 6 mmthick with a polymer film sandwiched therebetween.

FIG. 8 is a basic diagram of the detection device with three sensorsstuck in the corners making it possible to free the sides of the plate.

FIG. 9 shows the uncertainty in locating an impact at nine places on theplate of the device of FIG. 8 when the detection of the instant ofarrival of the wave packet is known to within 1 μs.

FIG. 10 is a diagrammatic view indicating the times of transit betweenan impact and the sensors of a detection system with four sensorsoperating in trios of sensors each functioning on a given quadrant ofthe plate.

FIG. 11 is a block diagram of a part of the internal architecture of theprogrammed component “wavepro4” of FIG. 5.

With reference firstly to FIG. 1, represented therein is a plate PLQcomprising four pairs PZT00, PZT10, PZT01, PZT11 of piezoelectrictransducers which each constitute an acoustic sensor, the twotransducers of each pair being fixed opposite one another on the twoopposite faces of the plate for example by sticking in order to gatherthe acoustic waves traveling around the plate. Unlike the case of theplate described in U.S. Pat. No. 9,816,229, the direction of theelectric polarizations and electric connections are respectivelysymmetric relative to the midplane of the plate and antiparallel orantisymmetric relative to the midplane and parallel in such a way as todiscourage any antisymmetric mode of propagation and to favor anysymmetric mode of propagation.

An orthogonal reference frame x, y with origin O is associated with theplate PLQ, the center of the plate being able to coincide with theorigin O of the reference frame. The four sensors constitute thevertices of a rectangle. The x and y axes cut the middles of the sides.PZT00, PZT10, PZT01, PZT11 have as respective coordinates (−p, −q) and(−p, q), (p, −q), (p, q). The acoustic waves can be generated by theshock of an object on the plate. The plate is an assemblage of at leasttwo isotropic plates each made from a rigid material constituting a goodisotropic acoustic conductor around 100 kHz such as glass. The platesare linked together rigidly by a polymer film such as polyvinyl butiral,denoted PVB. The thickness of the polymer film is of the order of 1 to 2millimeters. Its intrinsic acoustic impedance is small compared withthat of the glass so that in this account we shall continue to assume,as a first approximation, that each of the plates constituting thelaminated glass is capable of propagating symmetric and antisymmetricLamb waves. During the propagation in the laminated glass, the modespropagate in a plate and from one plate to another of the lamination. Ashock produced at the surface of the laminated glass gives rise in theplates to coupled symmetric modes and also to coupled antisymmetricmodes. Given the displacement vectors of matter which characterizesthem, the lower the acoustic frequency, the more the antisymmetric modesare attenuated by the PVB as compared with the symmetric modes. FIGS. 7a and 7 b clearly show this phenomenon of greater relative attenuationin respect of the antisymmetric modes: depicted therein are the signalsgathered independently by the transducers PZTa and PZTb of FIG. 6 afterselective amplification at 100 kHz. The signals of FIG. 7 a are aresponse to a shock generated 0.7 m away, while the signals of FIG. 7 bare a response to a shock generated 1.3 m away. Although moreeffectively generated than the symmetric modes, the more dispersivepropagation of the antisymmetric modes, combined with the attenuation bythe PVB, degrade the head of the antisymmetric wave packets faster. Thisis why when using laminated glass it is preferable to favor thedetection of the symmetric modes.

Furthermore, the applicant has observed that the arrangement of thesensors for the detection of the antisymmetric modes and the rejectionof the symmetric modes as described in patents 98/16229 and WO/11378 isunusable with laminated glasses. Specifically, as may be seen in thecurves of FIG. 7 c obtained via a shock on a laminated glass consistingof two plates of 6 mm glass rigidly bonded by a 2 mm PVB film accordingto the arrangement of FIG. 6, the S₀ modes of the upper and lower platesare no longer in full phase opposition, so that it is no longer possibleto discourage the S₀ mode effectively regardless of the intensity of theshock, according to the known methods described in the cited patents.

The aim of the present invention is to show how to detect the symmetricmodes reliably. To do this, the invention proposes to retain anarrangement with 2 facing transducers according to the symmetricarrangement and the parallel electrical connections of the transducersPZTa and PZTb of FIG. 6, so that this time the discouraging of theantisymmetric modes of larger amplitude near the receivers is ensured.Moreover, it is also necessary to take into consideration the fact thatthe intensity and the phase of the symmetric modes depend on the angleof impact on the glazing. The waveforms recorded by the receivers arethen extremely variable. For the antisymmetric modes, they decrease inamplitude and change shape according to the dispersion curvescharacterizing the Lamb modes, that is to say the mechanical wavesassociated with audible frequencies exhibit a smaller phase velocity andsmaller group velocity than the mechanical waves associated with higherultrasound frequencies. The effect of this is to renew the shape of thehead of the wave packet, thereby causing ever higher frequencycomponents to appear during propagation. Such is not the case for thesymmetric modes whose waveform is preserved since they are almostnondispersive for the ultrasound frequencies considered and the platethicknesses envisaged.

If we retain the signal processing described in French patent 98/16229which mentions a broadband amplification followed by a squaring followedby a peak detection and then an integration, it is apparent that thisprocess bases the detection of the instant of arrival of the wave packeton the obtaining of an energy threshold level. Now, the further away thesensor is from the position of the impact, the more it decreases inamplitude, hence implying a longer integration time before attainingthis threshold level. This integration time will then depend on theintensity of the shock, as well as on the nature and on the form of thepercussive object. This will be manifested at the end of the procedureby an uncertainty in the position of the impact.

To alleviate this drawback, the present invention proposes amodification of the process for detecting the instant of arrival of thewave packet. This modification consists in carrying out a measurement intwo steps. The first step is the same as the known method in U.S. Pat.No. 9,816,229, the principle of which has just been recalled. It makesit possible to determine in a first approximation the instant of arrivalof a packet and yields synchronization signals for the second step. Thesecond step, which is new, consists, for each sensor, in permanentlysampling the signal at the output of the selective amplifier centered ona high ultrasound frequency, around 100 kHz, and in saving the samplesin a FIFO (first in/first out) memory of sufficient depth to store theequivalent of 10 acoustic periods. Typically, for a sampling frequencyof one million samples per second (1 MSPS) and a central filterfrequency of 100 kHz, the depth of the FIFO memory will be 128 samples.The continuous sampling is performed independently for each pair ofsensors. The sampling of the signal is stopped by the synchronizationsignal. The content, then frozen, of the FIFO memory contains a digitalrecording of the head of the wave packet. Analysis of this content,performed further on in this description, yields the time intervalbetween the synchronization signal and the head of the wave packetmaking it possible to backtrack, independently of the amplitude or ofthe phase of the signal, to the instant of arrival of the head of thepacket. The accuracy in the measurement of this instant is now imposedonly by the sampling frequency, the signal-to-noise ratio and the numberof quantization bits.

During the first step, the receiver which is quickest to attain theenergy threshold for detecting the 100 kHz component defines the timeorigin and triggers a counting of the time until the wave packet reachesthe other receivers.

This time-stamping of events may possibly be called into question duringthe second step, in this instance when the difference in propagationtimes is very small.

The configuration of FIG. 1 is very suitable when one wishes to employ aplate with free edges. However, three pairs of sensors are sufficient todetermine the coordinates of an impact. FIG. 8 illustrates thisprinciple of detection with three sensors forming a right-angledtriangle. The axes of the physical reference frame as well as its originremain unchanged as compared with the configuration with four sensors ofFIG. 1. Unlike the transducers described in the above patents, thetransducers of the present invention have silver-plated lapoversenabling the connections to be made on the same face of the sensor andsubsequently enabling a fluid insulating adhesive to be used forsticking. The measurements of the differences in transit times of a wavepacket are preferably made in two orthogonal directions. Severalconfigurations with three sensors forming a right-angled triangle can beextracted from the four-sensor configuration of FIG. 1. The fourconfigurations described below lead to identical mathematical solutions,apart from the sign, for the coordinates of an impact (x_(r), y_(r)).Furthermore, each of these configurations is better suited than theothers, from the point of view of the accuracy of the measurement, if itis used only on a given quadrant of the plate. To appreciate this, letus take a configuration with three sensors PZT00, PZT10, PZT11corresponding to the diagram of FIG. 8 and let us look at theuncertainty which is obtained with regard to the position of the impactwhen the uncertainty in the arrival time of a wave packet is 1 μs for awave packet moving at 3350 m/s. FIG. 9 illustrates this uncertainty bysolid rectangles whose size, given in millimeters, is displayed at ninedifferent places on a plate of dimensions 1400 mm×800 mm. It is foundthat the uncertainty in the position of the impact remains less than 7mm² in the (1, 0) quadrant while it attains 80 mm² in the (0, 1)quadrant. It is therefore beneficial to restrict the use of theconfiguration of FIG. 8 to a single quadrant, the (1, 0) quadrant.However, the same accuracy can be obtained with regard to the otherquadrants if the detection trio is changed when the impact changesquadrant.

Thus, for a given impact, we firstly determine the quadrant (g_(y),g_(x)) to which it belongs, then we calculate the coordinates (x_(r),y_(r)) according to the formula associated with this quadrant.

FIG. 10 shows a rectangular plate with four pairs of sensors, withsilver-plated lapover, stuck in the corners, the silver-plated lapoversbeing oriented in such a way that the angular response of the sensors isas uniform as possible over an angular reception span of π/2. Thesensors also form a rectangle and make it possible to define a cartesianreference frame, the center of which is the center of the rectangleformed by the sensors and the axes of which pass through the middle ofthe sides, in a manner similar to FIG. 1. The cartesian coordinates ofthe sensors are (−p, −q), (−p, +q), (+p, +q), (+p, −q). The acousticwaves move at the velocity v. The times of propagation up to the sensorsof a wave packet generated by an impact at (x_(r), y_(r)) are t₀₀, t₀₁,t₁₀, t₁₁.

The coordinates (x_(r), y_(r)) are obtained by solving a system ofequations which is valid for a given quadrant. The four systems ofequations are:

 (g_(y), g_(x)).=(0,0). x_(r)<0 and y_(r)<0

$\begin{matrix}\begin{Bmatrix}{{v( {t_{01} - t_{00}} )} = {{v\quad\Delta\quad t_{xg}} = {{v\quad\Delta\quad t_{x0}} = {\sqrt{( {x_{r} - p} )^{2} + ( {y_{r} + q} )^{2}} - \sqrt{( {x_{r} + p} )^{2} + ( {y_{r} + q} )^{2}}}}}} \\{{v( {t_{10} - t_{00}} )} = {{v\quad\Delta\quad t_{yg}} = {{v\quad\Delta\quad t_{y0}} = {\sqrt{( {x_{r} + p} )^{2} + ( {y_{r} - q} )^{2}} - \sqrt{( {x_{r} + p} )^{2} + ( {y_{r} + q} )^{2}}}}}}\end{Bmatrix}\end{matrix}$  (g_(y), g_(x)).=(0,1). x_(r)<0 and y_(r)>0$\begin{matrix}\begin{Bmatrix}{{v( {t_{11} - t_{10}} )} = {{v\quad\Delta\quad t_{xg}} = {{v\quad\Delta\quad t_{x1}} = {\sqrt{( {x_{r} - p} )^{2} + ( {y_{r} - q} )^{2}} - \sqrt{( {x_{r} + p} )^{2} + ( {y_{r} - q} )^{2}}}}}} \\{{v( {t_{00} - t_{10}} )} = {{v\quad\Delta\quad t_{yg}} = {{v\quad\Delta\quad t_{y0}} = {\sqrt{( {x_{r} + p} )^{2} + ( {y_{r} + q} )^{2}} - \sqrt{( {x_{r} + p} )^{2} + ( {y_{r} - q} )^{2}}}}}}\end{Bmatrix}\end{matrix}$  (g_(y), g_(x)).=(1,0). x_(r)>0 and y_(r)<0$\begin{matrix}\begin{Bmatrix}{{v( {t_{00} - t_{01}} )} = {{v\quad\Delta\quad t_{xg}} = {{v\quad\Delta\quad t_{x0}} = {\sqrt{( {x_{r} + p} )^{2} + ( {y_{r} + q} )^{2}} - \sqrt{( {x_{r} - p} )^{2} + ( {y_{r} + q} )^{2}}}}}} \\{{v( {t_{11} - t_{01}} )} = {{v\quad\Delta\quad t_{yg}} = {{v\quad\Delta\quad t_{y1}} = {\sqrt{( {x_{r} - p} )^{2} + ( {y_{r} - q} )^{2}} - \sqrt{( {x_{r} - p} )^{2} + ( {y_{r} + q} )^{2}}}}}}\end{Bmatrix}\end{matrix}$  (g_(y), g_(x)).=(1,1). x_(r)>0 and y_(r)>0$\begin{matrix}\begin{Bmatrix}{{v( {t_{10} - t_{11}} )} = {{v\quad\Delta\quad t_{xg}} = {{v\quad\Delta\quad t_{x1}} = {\sqrt{( {x_{r} + p} )^{2} + ( {y_{r} - q} )^{2}} - \sqrt{( {x_{r} - p} )^{2} + ( {y_{r} - q} )^{2}}}}}} \\{{v( {t_{01} - t_{11}} )} = {{v\quad\Delta\quad t_{yg}} = {{v\quad\Delta\quad t_{y1}} = {\sqrt{( {x_{r} - p} )^{2} + ( {y_{r} + q} )^{2}} - \sqrt{( {x_{r} - p} )^{2} + ( {y_{r} - q} )^{2}}}}}}\end{Bmatrix}\end{matrix}$

The following formulae give the position of the impact (x_(r), y_(r)).It is sufficient to replace g_(x) and g_(y) by the value correspondingto the relevant quadrant. $\begin{matrix}{X_{r} = {{( {- 1} )^{g_{x}}\frac{\quad{\Delta\quad{txg}\{ {q\sqrt{p^{2}{v^{2}( {{4p^{2}} - {v^{2}\Delta\quad{txg}^{2}}} )}( {{4p^{2}} + {4q^{2}} - {v^{2}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}^{2}} )\Delta\quad{{tyg}^{2}( {{4q^{2}} - {v^{2}\Delta\quad{tyg}^{2}}} )}}} \}}}{4p\quad\Delta\quad{{tyg}( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{tyg}^{2}}} )}} )}}} +}} \\{( {- 1} )^{g_{x}}\frac{\quad{\underset{\quad}{\quad}\Delta\quad{txg}\quad p^{2}v^{2}\Delta\quad{{tyg}^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{{tyg}( {{{- \Delta}\quad{txg}} + {\Delta\quad{tyg}}} )}}} )}}}{4p\quad\Delta\quad{{tyg}( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{tyg}^{2}}} )}} )}}}\end{matrix}$ $\begin{matrix}{Y_{r} = {{( {- 1} )^{g_{y}}\frac{q\quad v^{2}\Delta\quad{{txg}( {{{- 4}p^{2}} + {v^{2}\Delta\quad{{txg}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}}} )}\Delta\quad{tyg}}{4( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}Ä\quad{tyg}^{2}}} )}} )}} +}} \\{( {- 1} )^{g_{y}}\frac{\sqrt{p^{2}{v^{2}( {{4p^{2}} - {v^{2}\Delta\quad{txg}^{2}}} )}( {{4p^{2}} + {4q^{2}} - {v^{2}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}^{2}} )\Delta\quad{{tyg}^{2}( {{4q^{2}} - {v^{2}\Delta\quad{tyg}^{2}}} )}}}{4( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}Ä\quad{tyg}^{2}}} )}} )}}\end{matrix}$

The determination of the measurement quadrant associated with an impactdepends on the differences in flight times between the four sensors.FIG. 4 illustrates the steps of measuring the difference in flight timesin the “y” direction on the basis of the sensors PZT00 and PZT10 and theacquisition card described according to the block diagram of FIG. 5. Thesignals represented are:

-   -   the signals output by the selective filters centered on 100 kHz,        FCH00 and FCH10,    -   the contents of the memories FIFO10 and FIFO00 at the instant of        low/high switching of the synchronization logic signals SYNCH10        and SYNCH00    -   the signal output by the squarer SQ00    -   the signal INTGR00 output by the integrator of the pathway        associated with PZT00,    -   the synchronization logic signals SYNC10 and SYNC00 output by        PMOS00 and PMOS10    -   the time interval Δtys₀ separating the synchronization signals    -   the time intervals TT10 and TT00 separating the heads of the        wave packets from the respective synchronization signals SYNC10        and SYNC00    -   a counter of counting frequency identical to the frequency of        sampling of the signals and triggered by the logic signal SYNC10

The signals relating to the other two sensors PZT01 and PZT11 are notrepresented, but yield similar signals on their own acquisition pathway.

In general and with reference to what has just been said, thedifferences in flight times between the sensors, these differences beinggiven as a number of clock periods XBUF, are determined according to thefollowing formulae and symbols:

-   #, designates a logical “or”,-   &, designates a logical “and”,-   abs, designates the absolute value,-   . designates multiplication,

A bar above a symbol designates the logical inverse. The symbols SP00 toSP11 are the outputs from D-type flip-flops associated with the signalsSYNC00 to SYNC11 and switching to the high logic state during a low/hightransition of the respective signals SYNC00 to SYNC11.

We have: $\begin{Bmatrix}{{\Delta\quad t_{x0}} = {{abs}\lbrack {{{{TT}_{01} - {TT}_{00} + {( {- 1} )^{{SS}_{x0}} \cdot {XBUF}}}\&}\quad\Delta\quad{txs}_{0}} \rbrack}} \\{{\Delta\quad{txs}_{0}} = ( {( {{{SP}_{01}\&}\quad\overset{\_}{{SP}_{00}}} )\quad\#\quad( {{\overset{\_}{{SP}_{01}}\&}\quad{SP}_{00}} )} )} \\{{\Delta\quad t_{x1}} = {{abs}\lbrack {{{{TT}_{11} - {TT}_{10} + {( {- 1} )^{{SS}_{x1}} \cdot {XBUF}}}\&}\quad\Delta\quad{txs}_{1}} \rbrack}} \\{{\Delta\quad{txs}_{1}} = ( {( {{{SP}_{11}\&}\quad\overset{\_}{{SP}_{10}}} )\quad\#\quad( {{\overset{\_}{{SP}_{11}}\&}\quad{SP}_{10}} )} )} \\{{SS}_{x0} = {{{SP}_{00}\&}\quad\overset{\_}{{SP}_{01}}}} \\{{SS}_{x1} = {{{SP}_{10}\&}\quad\overset{\_}{{SP}_{11}}}}\end{Bmatrix}$ $\begin{Bmatrix}{{\Delta\quad t_{y0}} = {{abs}\lbrack {{{{TT}_{10} - {TT}_{00} + {( {- 1} )^{{SS}_{y0}} \cdot {XBUF}}}\&}\quad\Delta\quad{tys}_{0}} \rbrack}} \\{{\Delta\quad{tys}_{0}} = ( {( {{{SP}_{10}\&}\quad\overset{\_}{{SP}_{00}}} )\quad\#\quad( {{\overset{\_}{{SP}_{01}}\&}\quad{SP}_{00}} )} )} \\{{\Delta\quad t_{y1}} = {{abs}\lbrack {{{{TT}_{11} - {TT}_{01} + {( {- 1} )^{{SS}_{y1}} \cdot {XBUF}}}\&}\quad\Delta\quad{tys}_{1}} \rbrack}} \\{{\Delta\quad{tys}_{1}} = ( {( {{{SP}_{11}\&}\quad\overset{\_}{{SP}_{01}}} )\quad\#\quad( {{\overset{\_}{{SP}_{11}}\&}\quad{SP}_{01}} )} )} \\{{SS}_{y0} = {{{SP}_{00}\&}\quad\overset{\_}{{SP}_{10}}}} \\{{SS}_{y1} = {{{SP}_{01}\&}\quad\overset{\_}{{SP}_{11}}}}\end{Bmatrix}$  g _(x)=0 if Δt _(y0) >Δt _(y1) and └TT ₁₀ −TT₀₀+(−1)^(SS) ^(y0) .XBUF&Δtys ₀┘<0or if Δt _(y1) >Δt _(y0) and [TT ₁₁ −TT ₀₁+(−1)^(SS) ^(y1) .XBUF&Δtys₁]<0g_(x)=1 otherwiseg _(y)=0 if Δt _(x0) ≧Δt _(x1) and └TT ₀₁ −TT ₀₀+(−1)^(SS) ^(x0).XBUF&Δtxs ₀┘<0or if Δt _(x1) ≧Δt _(x0) and [TT ₁₁ −TT ₁₀+(−1)^(SS) ^(x1) .XBUF&Δtxs₁]<0g_(y)=1 otherwise

The memories FIFO10 and FIFO00 each contain a digitization of the headof the wave packet detected by the respective sensors PZT10 and PZT00.The process for determining the coordinates in two steps shows here theimprovement which it affords: specifically, with the analog detectionsystem using an active integrator it was not possible to get the preciseinstant of arrival of the wave packet, simply because it is not possibleto know the time which the active integrator takes to switch from apositive saturated state of around +10 V, in the absence of any signal,to a negative saturated state of around −10 volts by integration of thesignal. During this transition, the signal drops at a given moment belowthe switching threshold THR of the PMOS transistor responsible foradapting this transition to logic levels compatible with the CMOS logic,characterized by a supply voltage VCC of possibly 5 volts. The switchingthreshold of the PMOS transistor is around 1.5 volts below VCC, i.e. 3.5volts. The transition time TT of the integrator is thus the time ofswitching from +10 V to +3.5 V triggered by the arrival of the wavepacket. This time depends on the amplitude of the envelope of thesquared signal SQ, on the gain afforded by the active integrator, andalso on characteristics peculiar to the operational amplifier used tomake the integrator. Although the integration time can be reduced byincreasing the gain of the amplifier and by reducing the positivevoltage of the saturated state, this arrangement is confronted with theproblem of the compromise that must be found between sensitivity andreliability. Too high a gain might make the integrator switch upon aparasitic signal, while too low a gain causes a loss of accuracy due toan integration time which varies too greatly between a low-intensityimpact and a high-intensity impact. By proceeding in two steps accordingto the present invention, the gain of the integrator is kept high enoughto keep good immunity to noise and the integration time corresponding tothe interval TT is known. Measurement of the interval TT consists forexample in rectifying the digitized signal, then in creating aninterpolation curve from the peak amplitudes of the rectified digitizedsignal. The intersection of the interpolation curve with the time axiscorresponding to the output value from the sampler in the absence of anysignal yields the origin instant t_(HD) of the packet. Another preferredsolution for measuring the interval TT consists in beginning from thesynchronization instant situated at an end of the window and insearching for the successive instants of zero-crossing toward the headof the packet. The instants of zero-crossing make it possible to lockonto the period of the digitized signal, while a test on the sum of theamplitudes between the zero-crossings, that is to say per half-period,makes a decision regarding the continuation of the algorithm. When themean value over a half-period is equal to the output value from thesampler in the absence of any signal, to within a threshold discrepancy,the algorithm is stopped and the mean value is regarded as being theorigin instant t_(HD) of the packet.

With reference now to FIG. 5, each sensor is associated with an analogacquisition pathway. The pathways associated with the sensors PZT00,PZT01, PZT11 are equivalent. The analog pathways are characterized by abroadband amplification A1 and A2 followed by a selective amplificationFCH in the ultrasound band, followed by a bypass, one pathway of whichgoes toward an 8-bit analog digital converter CAN with positivemeasurement span. The positive input voltages are obtained with the aidof a high-pass filtering and of a polarization bridge using thecomponents Ca, Ra, Rb, Rc. The converter is driven by control logicsignals CTA originating from a microcontroller μC. The converter CANsupplies the input of a FIFO memory. The memory data are transferred viaan 8-bit data bus DATA and other control signals CTF, ordering theloading, the unloading, the resetting to zero of the memory pointers,the placing of the output bus at high impedance, signals for indicatingthe state of fill of the FIFO memory, into RAM memory of the μC so as tobe processed there locally and/or to be transferred there onto anotherdevice or a more powerful computer such as a microcomputer via aparallel or serial communication port which may be a USB, MIDI, or RS232port. The logic levels between the microcontroller and the microcomputerare adapted with the aid of a logic level adapter LGCSHF. The otherpathway emanating from the bypass supplies a squaring stage followed byan envelope detection stage followed by an integrator stage supplying aPMOS transistor for adaptation to a CMOS logic level.

The pathway associated with the sensor PZT10 moreover possesses ahigh-voltage block for switching to emission mode. The high voltage isproduced by the module PMP shown explicitly by the diagram of FIG. 5Band comprising a diode pump D21 to D26 and capacitors C21 to C25supplied by the logic signal XHT whose high logic level is adapted tothe +12 V potential with the aid of the transistors T10 and T11 and thelow logic level of the −12 V potential with the aid of the transistorsT12 and T13. The signal XHT emanates from a logical “and” functionbetween the microcontroller system clock signal XBUF and the signal CGPPactivated at the high level when a plate integrity or ultrasoundpropagation speed measurement test procedure is triggered. Withoutvalidation by the signal CGPP the high-voltage module does not producethe high voltage HT of around 70 V. The switching block is managed bythe programmed logic component wavepro4 responsible for creating theexcitation logic burst and for counting the time of propagation of theacoustic waves between the sensor PZT10 and the other sensors. Theprogrammed logic component wavepro4 is driven by the microcontroller μC.The programmed counters are supplied via the same system clock frequencyXBUF as that of the microcontroller. This frequency is also the samplingfrequency of the analog digital converters CAN. The burst is obtainedwith the aid of the logic signals SRC and SNK responsible for orderingthe turning on of the switching transistors SWHTC and SWHTK. Thetransistors CMRC and CMSH are respectively responsible for placing thesensor in reception mode or for short-circuiting the input of the analogamplification pathway so as to protect it from the high voltage.

Part of the internal architecture of the programmed component wavepro4is described by the block diagram of FIG. 11. The component is furnishedwith D-type logic flip-flops FF1 to FF4 triggered by the synchronizationsignals SYNC00 to SYNC11. Logic combinations between the outputs ofthese flip-flops validate the flip-flops FF5 and FF6 whose outputs arethe signals SSx₀ and SSx₁, while other logic combinations define thelogic functions Δtxs₀ and Δtxs₁ representing the time intervals used forthe calculation of the difference in flight times between the sensors. Alogical “and” function between the clock signal XBUF and the functionsΔtxs₀ and Δtxs₁ respectively supply the 12-bit counters (Q0x₁₁ . . .Q0x₀) and (Q1x₁₁ . . . Q1x₀) associated with 3-state output registers,each register being identified and activated by the decoder of addresses(A3 . . . A0). The four high-order bits of the counters (Q0x₁₁ . . .Q0x₀) and (Q1x₁₁ . . . Q1x₀) share the same output register in thefollowing order high orders on the left: ((Q0x₁₁ . . . Q0x₈) and (Q1x₁₁. . . Q1x₈)). The logic flip-flops FF1 to FF4 and FF7, FF8 make itpossible in the same way to reproduce the logic functions SSy₀, SSy₁ andΔtys₀, Δtys₁, which via a logical “and” with the clock signal XBUFrespectively supply the 12-bit counters (Q0y₁₁ . . . Q0y₀) and (Q1y₁₁ .. . Q1y₀) also associated with 3-state output registers. The fourhigh-order bits of the counters (Q0y₁₁ . . . Q0y₀) and (Q1y₁₁ . . .Q1y₀) share the same output register in the following order, high orderson the left: ((Q0y₁₁ . . . Q0y₈) and (Q1y₁₁ . . . Q1y₈)).

All the output registers share the same 8-bit data bus DATA. Thecomponent also creates the logic functions IntHF and IntBF routed to theoutput pins of the component wavepro4 and producing when they switch tothe high level, an interrupt request detected by the microcontroller μCfurnished with inputs provided for this purpose. The function IntBF iscreated on the basis of the D-type logic flip-flop FFBF. The clock inputof the flip-flop originates from a selective amplifier stage FBFcentered on 10 kHz or preferably on the upper part of the audiblespectrum delivering a signal adapted to the CMOS logic by the transistorNMOS10. The flip-flop FFBF thus validates the presence of spectralenergy in the upper part of the audible spectrum. The logic functionIntHF is created on the basis of a logical “or” between the Q outputs ofthe flip-flops FF1 to FF4 validating the presence of spectral energy inthe ultrasound band toward 100 kHz. The time interval separating theinterrupts IntHF and IntBF characterizes an impact on the plate. Giventhe lower-frequency spectrum with which it is connected, the interruptIntBF always occurs after IntHF. When it does not occur or when itoccurs after overstepping a waiting time, the measurement is deniedsince it may have been caused by an untimely ultrasound signal beingpropagated to the plate via the floor. The {overscore (Q)} outputs ofthe logic flip-flops FF1 to FF4 are routed by way of a logical AND withthe signal XBUF to the output pins of the component wavepro4 and formthe respective clock signals LDCK00, LDCK01, LDCK10, LDCK11 of thememories FIFO00 to FIFO11. The FIFO memories are thus frozen at theinstant of switching of the signals SYNCij, with i and j equal to 0 or1.

The quantization of the intensity of the impact is carried out bydiverting the output signal from a high-frequency selective amplifier,that is to say around 100 kHz, for example that of FCH00 to a 12-bitimpact counter programmed in the component wavepro4 whose clock inputCLKi is the signal FCH00 adapted to the CMOS logic.

The microcontroller is preferably one with RISC architecture. Its systemclock XBUF is a multiple of the frequency 32768 Hz of the quartz QRTZ.The microcontroller is furnished with counters/timers, with severalinput/output ports operating with and without interrupt, with RAM randomaccess memory, with PROM read only or EPROM electrically programmableread only memory or with reprogrammable FLASH-type memory, with means ofin situ programming of the program code of JTAG type, and with means ofserial communication to other devices. It is endowed with at least fourcapture/compare functions for date-stamping the temporal events. Thesynchronization signals SYNC00, SYNC01, SYNC10, SYNC11 are in particularconnected to the capture/compare ports. The microcontroller is furnishedwith an arithmetic and logic unit allowing it to calculate the cartesiancoordinates of the impact, and to quantize the intensity of the impact.This solution is envisaged when the intensity information and positioninformation relating to the impact have to be transmitted rapidly. Inparticular, it is possible to use the acoustic plate as a piano or atwo-dimensional percussion instrument. A key then corresponds to a soundor an elementary audiovisual event executed in a predefined manner, whena given portion of the plate is struck with a greater or lesserintensity. In applications of this type, one seeks a fast response time,typically less than 10 milliseconds. The acoustic plate is of smallerdimensions, of the order of 0.25 m². The microcontroller is thenresponsible for all the processing and for communicating the information(x_(r), y_(r), impact counters) via the MIDI interface defined fordigital musical instruments.

According to another aspect which can be considered independently orotherwise of the aspects alluded to above, the invention proposes toimprove the ergonomics and the comfort of use of the glazing by dealingwith the problem of the sound nuisance caused by the impact of an objecton the plate according to a process for masking the sound generated bythe impact, with a synthesized sound triggered by IntHF. Specifically,the synchronization signals switch at the start of the audible soundgenerated by the percussion. This may therefore be advantageouslyexploited in order to trigger a recording followed by a processing andby a real-time sound synthesis which will be superimposed on the noiseof the impact and which will be able to enrich its frequency content insuch a way as to imitate a known sound, such as for example the noise ofa percussion instrument, of an animal or of a natural event or toproduce a sound which is simply different from the sound generated bythe impact. In certain regions of space the amplitude of the synthesizedsound will be able to oppose that of the sound produced by the impact insuch a way as to reduce the intensity of the noise. It is possible bychoice to condition the emission of the synthesized sound to thepresence of the interrupt IntBF occurring in practice less than 1millisecond after IntHF.

As has been said, the system according to the invention comprises acomputer which receives the signals emanating from the electricalprocessing circuits. The computer can, as a function of these signals,emulate the operation of certain peripherals such as for example apointer of mouse type or a keyboard. When the acoustic plate isassociated with a screen of large dimensions such as a plasma screen ora video projector projecting the graphical interface onto the surface ofthe acoustic plate, a homothetic correspondence can be establishedbetween the screen coordinates in pixels and the physical coordinates inmillimeters of any impact in such a way that a graphical pointer appearson the screen opposite the impact. This correspondence must beestablishable given that the relative positions of the video projectorand of the plate can change accidentally. For that purpose, theinvention proposes a procedure for calibrating the interface which issimple and fast. The procedure is carried out in five steps withreference to FIGS. 2- b, 2-c, 2-d, 2-e and 2-f. The operator makes surebeforehand and by orienting his video projector as required that theaxes of the graphical and physical cartesian reference frames arecolinear. The calibration procedure proper can then commence. FIG. 2- bis a screen for presenting the procedure. The operator must produce animpact on the plate in order to go to the step illustrated in FIG. 2- c.During this second step, a target appears on the screen. The screencoordinates N₀(i, j) of the target are known and close to the origin ofthe screen coordinates. The shock on the plate opposite the targetprovides the software with the corresponding cartesian physicalcoordinates N₀(x_(a), y_(c)). We then go to step 3 with FIG. 2- d. Asecond target appears with known screen coordinates N₁(k, l) close tothe maximum coordinates of the screen. Here again the operator isrequested to make an impact on the plate opposite the target so that thecalibration software determines the corresponding cartesian physicalcoordinates N₁(x_(b), y_(d)). The software is then furnished withsufficient information to determine the screen coordinates (N_(qx),N_(qy)) of any other impact from its physical coordinates (x_(r), y_(r))according to the following correspondence formula: $\begin{matrix}\begin{Bmatrix}{N_{qx} = {i + {( {k - i} )\frac{( {x_{r} - x_{a}} )}{( {x_{b} - x_{a}} )}}}} \\{N_{qy} = {j + {( {l - j} )\frac{( {y_{r} - y_{d}} )}{( {y_{c} - y_{d}} )}}}}\end{Bmatrix}\end{matrix}$

It remains to verify that the calibration is satisfactory. This is theaim of step 4 illustrated by FIG. 2- e: a target appears in the middleof the screen, for example with screen coordinates (400, 300) for ascreen displaying with a maximum SVGA resolution of (800, 600). Hereagain the operator is requested to make an impact opposite the target,this leading to step 5 illustrated by FIG. 2- f. A dialog window appearsdisplaying on one side the expected screen coordinates ATX and ATY, thatis to say (400, 300), and on the other the screen coordinates RESX andRESY deduced from the above correspondence formula. When the discrepancyoversteps a certain threshold, in practice by about 10 pixels, it isrecommended that the procedure be recommenced.

Once the homothetic correspondence between the plate and the graphicalscreen has been established, any impact at a given place on the plate,situated opposite the screen can be visualized on the screen via agraphical pointer. A software driver is then able to contrive matterssuch that these impacts are interpreted as events of another pointingperipheral, such as for example a peripheral of mouse type. An impact onthe plate will thus be interpreted as a mouse click or double click atthe place with the screen coordinates of the impact.

If these coordinates correspond to the location of an icon associatedwith the execution of a program, the latter will be executed.

The impacts may be interpreted as events associated with otherperipherals, in particular of keyboard type. This is very useful whennavigating around the Internet network and when wishing to communicateinformation requiring the input of alphanumeric characters.

Accordingly, the invention provides a floating and ever-accessible menubar represented in FIG. 3-B. This bar is disposed in a corner of thescreen. It contains a limited number of icons so as to mask the leastpossible graphical area. However, if despite this minimum footprint, thebar were to mask a background document, the bar can be moved to anothercorner of the screen indicated by the icon K03 representing a menu barassociated with an arrow indicating the corner in which the floatingmenu bar will be found during the next impact on this icon. Successiveimpacts on this icon will have the effect of moving the bar into thefour corners of the screen, the movement to another corner beingeffected in the counterclockwise direction.

The second visible icon K04 in FIG. 3-B represents a keyboard. An impactperformed opposite this icon triggers the appearance of the alphanumerickeyboard of FIG. 3-A. In order to maximize the area occupied by thekeys, so that the associating of an impact with a key is reliablewithout however covering the entire area of the screen, the keyboardcontains a restricted number of alphanumeric keys according to aconfigurable format, of the French AZERTY type or American QWERTY type.

The keyboard occupies the entire width of the screen, but only a thirdof its height. Here again, a supplementary key K01 is provided formoving the keyboard upward or downward should it mask the document ofinterest situated in the background. The key represents a keyboard withan arrow above or below depending on whether the keyboard is in thebottom or top part of the screen respectively.

Another aspect of the invention relates to the addition of afunctionality making it possible to limit the portion of the plate onwhich the impacts emulate the events of the mouse-type graphical pointer(click; double click etc.). Specifically, in a public arena it isdesirable to limit the field of action of somewhat unscrupulous users.In particular, the functionality is aimed at preventing a user fromquitting a software application by clicking in the closure icons or inthe drop-down menus. For this purpose it is sufficient to define a zoneof the screen authorizing the interpretation of impacts as events of themouse peripheral. An impact performed outside the authorized zone willoptionally trigger the displaying of a message. The message can take theform of a graphical window whose contours delimit the authorized zone.

The procedure for defining the authorized zone is illustrated in FIG. 2a. It shows a dialog window. The authorized zone is defined either bydirectly inputting the screen coordinates (X, Y) of the upper leftcorner, followed by the datum giving the width L and height H of thezone in pixels, or by direct acquisition of these data by successivelyactivating the “acquire” keys and by performing the acoustic impacts inthe corresponding upper left and lower right corners of the zone to bedefined. The impacts are thereafter converted into screen coordinates,from which are extracted the data which are displayed in the fieldsprovided for direct entry of the values.

This dialog window also contains a diagrammatic image of the platemaking it possible to configure the acquisition of the coordinates. Thesymbols p and q defining the cartesian coordinates of the sensorsthroughout this description are replaced in the figure by the symbols CHand CV respectively.

1. A device for acquiring the position coordinates of a source ofmechanical waves generated by an impact on the surface of a plate (PLQ)of finite dimensions comprising a set of acoustic sensors (PZT00 toPZT11) each formed of a pair of piezoelectric transducers (PZTa, PZTb)situated facing one another on either side of the plate, the devicecomprising processing means for determining the coordinates of thesource by analyzing the difference in propagation times of the acousticwaves generated by the source to the various sensors, a devicecharacterized in that, the processing means comprise in association witheach sensor (PZT00 to PZT11) a respective electronic circuit comprisingin cascade, means for digitizing the amplified signal around apredetermined frequency, and associated with means for limiting thedigitization to a time window starting before the acoustic waves reach asensor and ending after the acoustic waves have reached said sensor. 2.The device as claimed in claim 1, characterized in that the sensors arefour in number and the piezoelectric transducers of each sensor aredisks or wafers of piezoelectric ceramics stuck to either side of theplate, in such a way that the four sensors form on the plate thevertices of a rectangle whose center (O) constitutes the origin of thecoordinates of a cartesian reference frame whose x and y axes areparallel to at least two sides of the rectangle defined by the foursensors.
 3. The device as claimed in claim 1, characterized in that thedetermination of the position coordinates is achieved via a trio ofsensors taken from among the four sensors, said trio corresponding tothe three sensors nearest to the source, each trio being responsible fordetecting the coordinates in a given quadrant of the cartesian referenceframe defined by the sensors.
 4. The device as claimed in claim 3,characterized in that the locating of a point of interaction of thesource with the plate consists in extracting the ultrasound frequencycomponent in the vicinity of 100 kHz generated by the impact of a hardobject such as a fingernail, a metal key, a ballpoint pen, a hardplastic in the form of a rod and in determining the largest of thedifferences in absolute value of the times of flight between two sensorsof two first pairs (PZT00, PZT01) identified by gx=0 or (PZT10, PZT11)identified by gx=1, on the one hand, and two pairs of two sensors(PZT00, PZT10) identified by gy=0 and (PZT01, PZT11) identified by gy=1,on the other hand, so that the cartesian coordinates of the point ofimpact (xr, yr) on the plate are given by the formula: $\begin{matrix}{X_{r} = {{( {- 1} )^{g_{x}}\frac{\quad{\Delta\quad{txg}\{ {q\sqrt{p^{2}{v^{2}( {{4p^{2}} - {v^{2}\Delta\quad{txg}^{2}}} )}( {{4p^{2}} + {4q^{2}} - {v^{2}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}^{2}} )\Delta\quad{{tyg}^{2}( {{4q^{2}} - {v^{2}\Delta\quad{tyg}^{2}}} )}}} \}}}{4p\quad\Delta\quad{{tyg}( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{tyg}^{2}}} )}} )}}} +}} \\{( {- 1} )^{g_{x}}\frac{\quad{\underset{\quad}{\quad}\Delta\quad{txg}\quad p^{2}v^{2}\Delta\quad{{tyg}^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{{tyg}( {{{- \Delta}\quad{txg}} + {\Delta\quad{tyg}}} )}}} )}}}{4p\quad\Delta\quad{{tyg}( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}\Delta\quad{tyg}^{2}}} )}} )}}}\end{matrix}$ $\begin{matrix}{Y_{r} = {{( {- 1} )^{g_{y}}\frac{q\quad v^{2}\Delta\quad{{txg}( {{{- 4}p^{2}} + {v^{2}\Delta\quad{{txg}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}}} )}\Delta\quad{tyg}}{4( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}Ä\quad{tyg}^{2}}} )}} )}} +}} \\{( {- 1} )^{g_{y}}\frac{\sqrt{p^{2}{v^{2}( {{4p^{2}} - {v^{2}\Delta\quad{txg}^{2}}} )}( {{4p^{2}} + {4q^{2}} - {v^{2}( {{\Delta\quad{txg}} - {\Delta\quad{tyg}}} )}^{2}} )\Delta\quad{{tyg}^{2}( {{4q^{2}} - {v^{2}\Delta\quad{tyg}^{2}}} )}}}{4( {{q^{2}v^{2}\Delta\quad{txg}^{2}} + {p^{2}( {{{- 4}q^{2}} + {v^{2}Ä\quad{tyg}^{2}}} )}} )}}\end{matrix}$ or p and q designate the position of the sensors withrespect to the center O of the rectangle, v, the velocity of the platemode selected by the particular arrangement of the pair of transducersforming a sensor, Δtxg, (respectively Δtyg) the difference in thepropagation times of the wave packet generated by the impact between thesensors of one of the two first pairs (respectively of one of the nexttwo pairs), selected by the value of the index gx (respectively gy)equaling 0 if the coordinate yr (respectively xr) is negative and 0otherwise and being written Δtx₀ if gx=0 or Δtx₁ if gx=1 (respectivelyΔty₀ if gy=0 or Δty₁ if gy=1).
 5. The device as claimed in claim 1,characterized in that said electronic circuits associated with thesensors PZTij (i or j equaling 0 or 1) comprise in cascade two broadbandpreamplifier stages (A1ij, A2ij), a selective amplifier stage FCHijcentered on a frequency of around 100 kHz, a squaring stage (SQij), apeak detector stage (ENVLij), and an integrator stage (INTGRij), a stagePMOSij for adaptation to a logic level yielding a synchronizing signalSYNCij, said synchronization signal SYNCij triggering, via a logictransition, a flip-flop FFij responsible for ordering the stoppage ofthe analog digital converter CANij and the transferring into memoryFIFOij (first in-first out) of the digitized value of the signalemanating from the selective filter FCHij diverted to the converterCANij.
 6. The device as claimed in claim 1, characterized in that theprocessing means comprise downstream of said electronic circuitsassociated with the respective sensors a programmable logic module(wavepro4) driven by a microcontroller μC of an arithmetic and logicunit, of input/output ports operating on an interrupt basis, of RAMrandom access memory, of ROM type program memory, of a real-time clock,of ports for capturing the instants of switching of the signals SYNCij,of communication ports, of data buses and of address buses.
 7. Thedevice as claimed in claim 6, characterized in that the microcontrollerμC is furnished with software means for measuring the time interval TTijseparating the head of a wave packet t_(HDij) from the rising edge ofthe synchronization signal SYNCij, said software means consisting indetermining the instants of zero-crossing of the digitized signal on thebasis of the end of the digitization window commencing on the risingedge of SYNCij, while a test of decrease on the successive sum values ofthe amplitudes between the zero-crossings, that is to say a test on themean value of the signal per half-period, makes a decision regarding thecontinuation of the search algorithm for the instant t_(HDij). When themean value over a half-period is equal to the output value from thesampler in the absence of any signal, to within a threshold discrepancy,the algorithm is stopped, and the mean value is regarded as being theorigin instant t_(HDij) of the packet.
 8. The device as claimed in claim4, characterized in that the values of the bits gx and gy are determinedby the following formulae: $\begin{matrix}\begin{Bmatrix}{{\Delta\quad t_{x0}} = {{abs}\lbrack {{{{TT}_{01} - {TT}_{00} + {( {- 1} )^{{SS}_{x0}} \cdot {XBUF}}}\&}\quad\Delta\quad{txs}_{0}} \rbrack}} \\{{\Delta\quad{txs}_{0}} = ( {( {{{SP}_{01}\&}\quad\overset{\_}{{SP}_{00}}} )\#( {{\overset{\_}{{SP}_{01}}\&}\quad{SP}_{00}} )} )} \\{{\Delta\quad t_{x1}} = {{abs}\lbrack {{{{TT}_{11} - {TT}_{10} + {( {- 1} )^{{SS}_{x1}} \cdot {XBUF}}}\&}\quad\Delta\quad{txs}_{1}} \rbrack}} \\{{\Delta\quad{txs}_{1}} = ( {( {{{SP}_{11}\&}\quad\overset{\_}{{SP}_{10}}} )\#( {{\overset{\_}{{SP}_{11}}\&}\quad{SP}_{10}} )} )} \\{{SS}_{x0} = {{{SP}_{00}\&}\quad\overset{\_}{{SP}_{01}}}} \\{{SS}_{x1} = {{{SP}_{10}\&}\quad\overset{\_}{{SP}_{11}}}}\end{Bmatrix} \\\begin{Bmatrix}{{\Delta\quad t_{y0}} = {{abs}\lbrack {{{{TT}_{10} - {TT}_{00} + {( {- 1} )^{{SS}_{y0}} \cdot {XBUF}}}\&}\quad\Delta\quad{tys}_{0}} \rbrack}} \\{{\Delta\quad{tys}_{0}} = ( {( {{{SP}_{10}\&}\quad\overset{\_}{{SP}_{00}}} )\#( {{\overset{\_}{{SP}_{10}}\&}\quad{SP}_{00}} )} )} \\{{\Delta\quad t_{y1}} = {{abs}\lbrack {{{{TT}_{11} - {TT}_{01} + {( {- 1} )^{{SS}_{y1}} \cdot {XBUF}}}\&}\quad\Delta\quad{tys}_{1}} \rbrack}} \\{{\Delta\quad{tys}_{1}} = ( {( {{{SP}_{11}\&}\quad\overset{\_}{{SP}_{01}}} )\#( {{\overset{\_}{{SP}_{11}}\&}\quad{SP}_{01}} )} )} \\{{SS}_{y0} = {{{SP}_{00}\&}\quad\overset{\_}{{SP}_{10}}}} \\{{SS}_{y1} = {{{SP}_{01}\&}\quad\overset{\_}{{SP}_{11}}}}\end{Bmatrix}\end{matrix}$  g _(x)=0 if Δt _(y0) >Δt _(y1) and └TT ₁₀ −TT₀₀+(−1)^(SS) ^(y0) .XBUF&Δtys ₀┘<0or if Δt _(y1) >Δt _(y0) and [TT ₁₁ −TT ₀₁+(−1)^(SS) ^(y1) .XBUF&Δtys₁]<0g_(x)=1 otherwiseg _(y)=0 if Δt _(x0) ≧Δt _(x1) and └TT ₀₁ −TT ₀₀+(−1)^(SS) ^(x0).XBUF&Δtxs ₀┘<0or if Δt _(x1) ≧Δt _(x0) and [TT ₁₁ −TT ₁₀+(−1)^(SS) ^(x1) .XBUF&Δtxs₁]<0g_(y)=1 otherwise
 9. The device as claimed in claim 1, characterized inthat the acoustic plate is a laminated glass consisting of an assemblageof plates of like thickness, stuck together by a polymer film.
 10. Thedevice as claimed in claim 1, characterized in that the piezoelectrictransducers of a sensor are ferroelectric ceramics whose polarizationvectors are oriented symmetrically with respect to the thickness of theplate and whose electrical connections are in parallel.
 11. The deviceas claimed in claim 1, characterized in that the piezoelectrictransducers of a sensor are ferroelectric ceramics whose polarizationvectors are oriented antisymmetrically with respect to the thickness ofthe plate and whose electrical connections are in antiparallel.
 12. Thedevice as claimed in claim 1, characterized in that the piezoelectricceramics are disks or plates whose lower electrode, in contact with theplate, is brought to a small upper face portion, while remaininginsulated from the upper electrode by an electrical insulating guardstrip.
 13. The device as claimed in claim 1, characterized in that oneof the sensors for example PZT10 is able to be switched into an emitterof an ultrasound wave packet so as to trigger a measurement of velocityof propagation of the acoustic waves in at least two differentdirections given by the positions of the other sensors.
 14. The deviceas claimed in claim 1, constituting a peripheral interface with acomputer fitted with a screen.
 15. The device as claimed in claim 14,characterized in that the acoustic plate also serves as a display screenfor visualization by scattering of projected light, either by frostingat least one of the faces of the glass plates, or by using a translucentpolymer film, optionally colored and optionally combined with an effectof light concentration by means of a Fresnel lens.
 16. The device asclaimed in claim 14, characterized in that the axes of the screenreference frame and of the acoustic plate are colinear.
 17. The deviceas claimed in claim 14, characterized in that a homotheticcorrespondence between a pixel (N_(qx), N_(qy)) of the screen referenceframe and a physical point (x_(r), y_(r)) of the plate opposite thegraphical pixel is established by automated calibration according to thefollowing operations: displaying by the software of a target at variouspositions with known screen coordinates and measuring of thecorresponding physical coordinates.
 18. The device as claimed in claim1, characterized in that the acoustic plate constitutes a graphicalpointing peripheral capable of emulating another pointing peripheralsuch as for example a peripheral of mouse type, an impact on the plateat a given position then being interpreted according to a particularcoding, as a click or a double click carried out on the correspondingscreen coordinates and triggering the execution of programs associatedwith an icon situated opposite the impact.
 19. The device as claimed inclaim 8, characterized in that the zone of emulation of the mouse events(click, double click, etc.) is limited to an authorized portion of thescreen zone exhibiting the form of a rectangle defined by the X, Ycoordinates in pixels of one of its corners as well as its width L andits height H in pixels, it being possible for these values to be entereddirectly at the keyboard or to be deduced by acquiring the coordinatesof the impacts in the corners of the authorized zone to be defined. 20.The device as claimed in claim 18, characterized in that it is furnishedwith software means making it possible to produce a floating toolbar,permanently accessible, consisting of several icons K03, K04 ensuringwhen they are struck by an impact: the appearance (K04) on the screen ofan alphanumeric keyboard, two of whose keys K01 and K02 make provisionrespectively for its upward movement and its reduction to the floatingmenu bar, the fast and circular movement (K03) of the toolbar into oneof the four corners of the screen, designated by the direction of thearrow represented on the icon.
 21. The device as claimed in claim 1,characterized in that it is furnished with software means such that theportion of the plate which does not serve as a screen is alsointeractive and is configured as an extension of the screen zone. 22.The device as claimed in claim 1, characterized in that it is furnishedwith software means such that the portion of the plate which serves as ascreen is regarded as a special zone making it possible to quit or toswitch from any software application managing the execution of groups ofprograms associated with various zones of the plate which are situatedoff-screen.
 23. The device as claimed in claim 14, characterized in thatit is furnished with software means such as client/server type protocolsallowing the graphical interface to be connected, via a modem or anetwork card, to an Internet access provider.
 24. The device as claimedin claim 14, characterized in that it contains software means making itpossible to update the multimedia content (picture, sound, video)available on the host computer of the graphical interface from a remotecomputer.
 25. The device as claimed in claim 14, characterized in thatthe electronic circuits associated with the respective sensors PZTijcomprise downstream of said broadband amplification means A2ij a bypassto means of frequency enrichment of the audible acoustic signalgenerated by the impact on the plate, as well as means for reconvertingthe enriched signal into an analog signal and sending it to loudspeakersso as to mask the nuisance caused by the impact in the form of adifferent sound reproducing for example the noise of a percussioninstrument within a symphonic composition, or the noise of an animal orof a natural event, said means of enrichment being implemented at thevery instant IntHF at which the first of the four synchronizationsignals SYNCij switches logic level.
 26. The device as claimed in claim14, characterized in that the acoustic plate constitutes a graphicalpointing peripheral capable of emulating another pointing peripheralsuch as for example a peripheral of mouse type, an impact on the plateat a given position then being interpreted according to a particularcoding, as a click or a double click carried out on the correspondingscreen coordinates and triggering the execution of programs associatedwith an icon situated opposite the impact.
 27. The device as claimed inclaim 26, characterized in that the zone of emulation of the mouseevents (click, double click, etc.) is limited to an authorized portionof the screen zone exhibiting the form of a rectangle defined by the X,Y coordinates in pixels of one of its corners as well as its width L andits height H in pixels, it being possible for these values to be entereddirectly at the keyboard or to be deduced by acquiring the coordinatesof the impacts in the corners of the authorized zone to be defined. 28.The device as claimed in claim 26, characterized in that it is furnishedwith software means making it possible to produce a floating toolbar,permanently accessible, consisting of several icons K03, K04 ensuringwhen they are struck by an impact: the appearance (K04) on the screen ofan alphanumeric keyboard, two of whose keys K01 and K02 make provisionrespectively for its upward movement and its reduction to the floatingmenu bar, the fast and circular movement (K03) of the toolbar into oneof the four corners of the screen, designated by the direction of thearrow represented on the icon.
 29. The device as claimed in claim 14,characterized in that it is furnished with software means such that theportion of the plate which does not serve as a screen is alsointeractive and is configured as an extension of the screen zone. 30.The device as claimed in claim 14, characterized in that it is furnishedwith software means such that the portion of the plate which serves as ascreen is regarded as a special zone making it possible to quit or toswitch from any software application managing the execution of groups ofprograms associated with various zones of the plate which are situatedoff-screen.
 31. The device as claimed in claim 5, characterized in thatthe electronic circuits associated with the respective sensors PZTijcomprise downstream of said broadband amplification means A2 ij a bypassto means of frequency enrichment of the audible acoustic signalgenerated by the impact on the plate, as well as means for reconvertingthe enriched signal into an analog signal and sending it to loudspeakersso as to mask the nuisance caused by the impact in the form of adifferent sound reproducing for example the noise of a percussioninstrument within a symphonic composition, or the noise of an animal orof a natural event, said means of enrichment being implemented at thevery instant IntHF at which the first of the four synchronizationsignals SYNCij switches logic level.